Control devices of that kind are already known and in order to initiate them via a supply terminal an operating voltage from the 12V electrical system of the motor vehicle is supplied. Then the control functions are implemented in a so-called micro-controller that typically needs only a voltage of 5V for the operation. In this case the control device has a voltage regulator (DC/DC down converter) in order to convert the voltage of the electrical system from 12V to 5V. As long as the voltage of the electrical system is higher than approx. 6V, the voltage regulator can provide on its output the necessary voltage for the operation of the micro-controller.
Assigned electrical components have to be appropriately triggered when the internal combustion engine of a motor vehicle is started. When the motor is started, the control device is typically supplied via a terminal of the ignition lock with the voltage of the electrical system from a starter battery of the motor vehicle and then the voltage regulator in the control device supplies the micro-controller with 5V. After a short initialization phase the micro-controller starts its regular operation and activates for example a magnetic switch to turn on (starting and generating current) the starter. In case of an internal combustion engine with direct injection, a gasoline pump is started at the same time in order to build up the necessary fuel pressure for the first injection pulse.
It is problematic to carry out control functions when the motor is started, because at that time the necessary operating voltage to operate the control device cannot be provided by an electric generator assigned to the internal combustion engine but only via the starter battery of the motor vehicle. Bringing up the motor to its starting speed via the starter requires a considerable electrical power that has to be provided by the starter battery. The electric current that is taken up from the starter at the beginning of the starting procedure is especially high. In this moment the current is only limited by the internal resistance of the starter battery and the resistors of the feed lines and the starter winding. A counter-electromotive force that reduces the current is only generated with the run-up of the starter. When the motor is started, a strong drop of the voltage on the battery terminals occurs due to the internal resistance of the starter battery that is different from zero.
This temporary drop of the battery voltage can cause that the operating voltage supplied to the control device is temporarily not sufficient to reliably carry out the control functions. This problem is especially increased through a weakening of the starter battery caused by old age or through low temperatures. It turned out that the battery voltage (nominal 12V) can drop for example for a period of 100 ms to values of approx. 3V when the motor is started. In this case the operating voltage (if necessary down-converted) supplied to the control device is not sufficient anymore to guarantee a reliable operation of the control device, especially the operation of a possibly provided micro-controller.